Chapter 13 The Respiratory System
The trillions of cells in the body require an abundant and continuous supply of
oxygen to carry out their vital functions. We cannot "do without oxygen" for even a
little while, as we can without food or water. Furthermore, as cells use oxygen, they
give off carbon dioxide, a waste product the body must get rid of.
The cardiovascular and respiratory systems share responsibility for supplying the
body with oxygen and disposing of carbon dioxide. The respiratory system organs
oversee the gas exchanges that occur between the blood and the external
environment. The transportation of respiratory gases between the lungs and the
tissue cells is accomplished by the cardiovascular system organs, using blood as the transporting fluid. If
either system fails, body cells begin to die from oxygen starvation and accumulation of carbon dioxide.
Functional Anatomy of the Respiratory System
 Name the organs forming the respiratory passageway from the nasal cavity to the alveoli of the
lungs (or identify them on a diagram or model) and describe the function of each.
o Nose – air enters the nose by passing through the external nares , nostril, into the
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nasal cavity, which is divided by a midline nasal septum
 Olfactory receptors are located in the mucosa in the slit-like superior part of
the nasal cavity
 Respiratory mucosa rests on a rich network of thin-walled veins that
warms the air as it flows past
 Mucosa glands moisten the air and traps incoming bacteria and other foreign
debris
 Ciliated cells of the nasal mucosa create a gentle current that moves
contaminated mucus posteriorly toward the throat where it is swallowed and
digested
 Conchae – mucosa-covered projections or lobes that greatly increase the
surface area of the mucosa exposed to the air
Pharynx – muscular passageway (throat) that serves as a common passageway
for food and air – is continuous with the nasal cavity anteriorly via the internal
nares
 Air enters the superior portion, the nasopharynx from the nasal cavity and
then descends through the oropharynx and laryngopharynx to enter the
larynx below
 Food enters the mouth and then travels along with air through the
oropharynx and the laryngopharynx but is directed into the
esophagus instead of entering the larynx
 Tonsils – small masses of lymphatic tissue that ring the pharynx, where
they are found in the mucosa – function is to trap and remove any bacteria
or other foreign pathogens entering the throat
Larynx – voice box – routes air and food into the proper channels and plays a role
in speech
 Thyroid cartilage – protrudes anteriorly and is commonly called the Adam’s
apple – the largest of the hyaline cartilages and is shield-shaped
 Epiglottis – protects the superior opening of the larynx – when we swallow
food or fluids, the larynx is pulled upward and the epiglottis tips, forming a
lid over the opening of the larynx routing the food into the esophagus
If anything other than air enters the larynx, a cough reflex is
triggered to expel the substance
Trachea – windpipe – lined with ciliated mucosa with cilia that beat continuously
and in a direction opposite to that of the incoming air to propel mucus, loaded with
dust particles and other debris, away from the lungs to the throat, where it can be
swallowed or spat out
Primary bronchi – right and left formed by the division of the trachea – the right
is wider, shorter, and straighter leaving foreign objects more likely to become
lodged here
 By the time incoming air reaches the bronchi, it is warm, cleansed of most
impurities, and well humidified
 Smaller subdivisions of the primary bronchi within the lungs are direct routes
to the air sacs
Lungs – contain the alveoli, air sacs, where gas exchange occurs
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 Describe several protective mechanisms of the respiratory system.
o See items above and below dealing with cilia, mucus, macrophages, coughing reflex,
lymphatic tissue, and alveolar pores
 Describe the structure and function of the lungs and the pleural coverings.
o A paired set that occupies the entire thoracic cavity except for the most central area, the
mediastinum, which houses the heart, the great blood vessels, bronchi, esophagus, and
other organs
 The narrow superior portion of each lung, the apex, is located just deep to the
clavicle
 The broad lung area resting on the diaphragm is the base
 Each lung is divided into lobes by fissures, the left lung has two lobes while the right
lung has three
o The surface of each lung is covered with a visceral serosa called the pulmonary, or
visceral, pleura and the walls of the thoracic cavity are lined by the parietal pleura
 Pleural membranes produce pleural fluid, a slippery serous secretion which allows
the lungs to glide easily over the thorax wall during breathing movements and
causes the two pleural layers to cling together
 The lungs are held tightly to the thorax wall, and the pleural space is more of a
potential space than an actual one
o After the primary bronchi enter the lungs, they subdivide into smaller and smaller branches,
finally ending in the smallest of the conducting passageways, the bronchioles resulting in a
respiratory tree with all but the smallest branches having reinforcing cartilage in their
walls
 Terminal bronchioles lead into respiratory zone structures, even smaller conduits
that eventually terminate in alveoli, or air sacs – the respiratory zone (respiratory
bronchioles, alveolar ducts, alveolar sacs, and alveoli) is the only site of gas
exchange while all other respiratory passages are conducting zone structures that
serve as conduits to and from the respiratory zone
 The clusters of alveoli make up the bulk of the lungs leaving most of the lungs as air
space and the rest of the tissue as stroma, an elastic connective tissue
o The walls of the alveoli are a single, thin layer of squamous epithelial cells with alveolar
pores connecting neighboring air sacs providing alternate route for air to reach alveoli
whose feeder bronchioles have been clogged by mucus or are blocked
 The external surfaces of the alveoli are covered with a cobweb of pulmonary
capillaries and together, the alveolar and capillary walls and their fused basement
membranes construct the respiratory membrane (air-blood barrier), which has
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air flowing past on one side and blood flowing past on the other with gas exchange
occurring by simple diffusion
Macrophages wander in and out of the alveoli picking up bacteria, carbon particles,
and other debris
Chunky cuboidal cells present in amongst the epithelial cells produce a lipid
molecule called surfactant, which coats the gas-exposed alveolar surfaces and
lowers the surface tension of the film of water lining each alveolar sac so that the
alveoli do not collapse between each breath
Respiratory Physiology
 Define cellular respiration, external respiration, internal respiration, pulmonary ventilation,
expiration, and inspiration.
o The major function of the respiratory system is to supply the body with oxygen and to
dispose of carbon dioxide in a process that has four steps collectively termed respiration
 Pulmonary ventilation – breathing – air moving into and out of the lungs so gases
in the air sacs of the lungs are continuously changed and refreshed
 External respiration – gas exchange between the pulmonary blood and alveoli
 Respiratory gas transport – gases must be transported to and from the lungs and
tissue cells of the body via the bloodstream
 Internal respiration – at the capillary level, gas exchange must be made between
the blood and tissue cells – the use of oxygen and production of carbon dioxide by
tissue cells is called cellular respiration
 Explain how the respiratory muscles cause volume changes that lead to air flow into and out of the
lungs (breathing).
o Breathing is a mechanical process that depends on volume changes occurring in the
thoracic cavity
o Volume changes lead to pressure changes, which lead to the flow of gasses to
equalize the pressure
o Occurs in two phases – inspiration and expiration
 Inspiration – the diaphragm and external intercostals contract and the size of
the thoracic cavity increases
 As the diaphragm contract, it moves inferiorly and flattens out
 The superior-inferior dimension of the thoracic cavity increases
 Contraction of the external intercostals lifts the rib cage and thrusts the
sternum forward, which increases the anteroposterior and lateral
dimensions of the thorax
 As intrapulmonary volume increases, the gases within the lungs spread
out to fill the larger space
 The decrease in the gas pressure in the lungs produces a partial vacuum,
which sucks air into the lungs and air continues to move into the lungs until
the intrapulmonary pressure equals atmospheric pressure
 Expiration – largely a passive process that depends on the natural elasticity of the
lungs instead of on muscle contraction
 As the inspriatory muscles relax and resume their initial resting length, the
rib cage descends and the lungs recoil
 Both the thoracic and intrapulmonary volumes decrease
 As the intrapulmonary volume decreases, the gases inside the lungs are
forced more closely together and the intrapulmonary pressure rises to a point
higher than atmospheric pressure
This causes the gases to flow out to equalize the pressure inside and outside
the lungs
 Under normal conditions, expiration is effortless, but if the respiratory passageways
are narrowed by spasms of the bronchioles or clogged with mucus or fluid, expiration
becomes an active process - forced expiration – the internal intercostal muscles
are activated to help depress the rib cage, and the abdominal muscles contract and
help force air from the lungs by squeezing the abdominal organs upward against the
diaphragm
 The normal pressure within the pleural space (intrapleural pressure) is always
negative, and this is the major factor preventing collapse of the lungs
Define the following respiratory volumes: tidal volume, vital capacity, expiratory reserve volume,
inspiratory reserve volume, and residual air.
o Tidal volume – respiratory capacity – the amount of air inhaled or exhaled with a
normal breath
o Vital capacity – the sum of the TV + IRV + ERV – the volume of air that can be expelled
from the lungs by forcible expiration after the deepest inspiration, total exchangeable air
o Expiratory reserve volume – the amount of air that can be forcibly exhaled after a tidal
expiration
o Inspiratory reserve volume – the amount of air that can be taken in forcibly over the
tidal volume
o Residual air – the air that still remains in the lungs that cannot be voluntarily expelled –
important because it allows gas exchange to go on continuously even between breaths and
helps to keep the alveoli open/inflated
Name several nonrespiratory air movements and explain how they modify or differ from normal
respiratory air movements.
o Coughs – act to clear the lower respiratory passageways – clear the air passages of debris
or collected mucus – clears the lower respiratory passageways
o Sneezes – similar to cough, except that expelled air is directed through the nasal cavities –
clear the air passages of debris or collected mucus – clears the upper respiratory passages
o Crying and laughing – inspiration followed by release of air in a number of short breaths
– primarily an emotional induced mechanism
o Hiccups – sudden inspirations resulting from spasms of diaphragm initiated by irritation of
the diaphragm or phrenic nerves, which serve the diaphragm
o Yawn – very deep inspiration, taken with jaws wide open – ventilates all alveoli – not
necessarily triggered by the need to increase the amount of oxygen in blood
Describe the process of gas exchanges in the lungs and tissues.
o All gas exchanges are made according to the laws of diffusion, that is, the movement occurs
toward the area of lower concentration of the diffusing substance
 In external respiration the blood picks up oxygen from the lungs while dropping off
carbon dioxide to be exhaled
 At the systemic circulation level, oxygen is taken up by cells for use while carbon
dioxide is released from the cells into the blood
Describe how oxygen and carbon dioxide are transported in the blood.
o Oxygen is transported in the blood in two ways
 Most is attached to hemoglobin molecules inside the RBCs to form oxyhemoglobin
– HbO2
 A small amount of oxygen is carried dissolved in the plasma
o Carbon dioxide is transported in two ways
 Within the plasma as the bicarbonate ion (HCO3-), which plays a very important
role in the blood buffer system
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Before carbon dioxide can diffuse out of the blood into the alveoli, it first must
be released from its bicarbonate ion form by combining with hydrogen ions
(H+) to form carbonic acid (H2CO3), which quickly splits to form water and
carbon dioxide allowing the carbon dioxide to diffuse from the blood and enter
the alveoli
 A smaller amount is carried inside the RBCs bound to hemoglobin
Name the brain areas involved in control of respiration.
o The activity of the respiratory muscles, the diaphragm and external intercostals, is
regulated by nerve impulses transmitted to them from the brain by the phrenic and
intercostal nerves
o The neural centers that control respiratory rhythm and depth are located in the medulla
and pons
 The medulla, which sets the basic rhythm of breathing, contains a self-exciting
inspiratory center, as well as other respiratory centers
 The pons centers appear to smooth out the basic rhythm of inspiration and
expiration set by the medulla
 The communication between the pons and medulla centers maintains the
normal respiratory rate called eupnea
o The bronchioles and alveoli have stretch receptors that respond to over-inflation
through impulses sent from the stretch receptors to the medulla by the vagus nerves to
end inspiration and begin expiration
 During exercise, breathing becomes more vigorous and deep because the brain
centers send more impulses to the respiratory muscles resulting in a respiratory
pattern called hyperpnea
Name several physical factors that influence respiratory rate.
o Physical factors – talking, coughing, exercise, and increased body temperature can
increase the rate of breathing
o Volition (conscious control) – singing, swallowing, breath control such as holding our
breath
o Emotional factors – being scared, gasping result from reflexes initiated by emotional
stimuli acting through centers in the hypothalamus
o Chemical factors – levels of carbon dioxide and decreased blood pH lead to an increase in
the rate and depth of breathing by acting directly on the medulla centers while levels of
oxygen in the blood is detected by chemoreceptor regions in the aorta and carotid
artery that send impulses to the medulla when blood oxygen levels are dropping
Explain the relative importance of oxygen and carbon dioxide in modifying the rate and depth of
breathing.
o The concentration of carbon dioxide in the blood is the more important stimulus for
breathing in a healthy person while decreasing oxygen levels only becomes important when
they are dangerously low
o In people with emphysema and chronic bronchitis, increased levels of carbon dioxide
are no longer recognized as important by the brain and dropping oxygen levels become the
respiratory stimulus, which is why these people are only given low levels of oxygen,
otherwise they would stop breathing
Explain why it is not possible to stop breathing voluntarily.
o The will to hold your breath can be overridden by respiratory centers when oxygen
supplies are low or blood pH is falling – involuntary controls take over and normal
respiration will begin again
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 Define apnea, dyspnea, hyperventilation, hypoventilation, and chronic obstructive pulmonary
disease (COPD).
o Apnea – cessation of breathing often caused by hyperventilation, which is often brought on
by anxiety attacks
o Dyspnea – difficult or labored breathing, often referred to as “air hunger”
o Hyperventilation – when breathing becomes more deep and more rapid and is separate
from hyperpnea of exercise – blows off more carbon dioxide and decreases the amount of
carbonic acid, which returns blood pH to the normal range
o Hypoventilation – extremely slow or shallow breathing that allows carbon dioxide levels to
increase in the blood and bring blood pH back to normal when it becomes too alkaline, or
basic
o Chronic obstructive pulmonary disease (COPD) – the chronic obstructive pulmonary
disease exemplified by chronic bronchitis and emphysema,
Respiratory Disorders
 Describe the symptoms and probable causes of COPD and lung cancer.
o COPD patients
 Have four things in common
 History of smoking – or exposure to air pollution
 Dyspnea, which becomes progressively more severe
 Coughing and frequent pulmonary infections
 Hypoxia – retention of carbon dioxide and respiratory acidosis
 Smoking or exposure to air pollution can lead to continual bronchial irritation and
inflammation, which leads to chronic bronchitis with excessive mucus production,
coughing, and bronchospasms and/or it can lead to the breakdown of elastin in
connective tissue of lungs, which leads to emphysema and the destruction of
alveolar walls, lung fibrosis, and air trapping
 Both can lead to airway obstruction or air trapping, dyspnea, and frequent
infections
 Ultimately lead to respiratory failure
o Lung cancer
 Typically caused by smoking
 Which increases the heart rate, constricts peripheral blood vessels throughout
the body, disrupts the flow of air in the lungs, and affects the brain and mood
and ultimately to atherosclerosis and heart disease, strokes, cataracts, and
early onset of osteoporosis
 Which causes continuous irritation in the lungs causing an increase in mucus
production but slowed cilia movements so mucus and irritants cannot be
cleared from the lungs and decreases the activity of lung macrophages
 Which can cause the pooling of mucus in the lower respiratory tree and
increased frequency of pulmonary infections including pneumonia and COPD
 Ultimately, it is the irritating effects of the cocktail of free radicals and other
carcinogens in tobacco smoke that leads to lung cancer
 Especially nitrosamine and the tars in tobacco that contain
carcinogens that cause the epithelial cells lining the bronchial tree to
proliferate wildly and lose their normal structure
 Three main types of lung cancer
 Squamous cell carcinoma
 Adenocarcinoma
 Small cell carcinoma
Developmental Aspects of the Respiratory System
 Describe normal changes that occur in respiratory system functioning from infancy to old age.
o In the fetus, the lungs are filled with fluid and all respiratory exchanges are made by the
placenta
o At birth, the fluid-filled pathway is drained, and the respiratory passageways fill with air
and the alveoli inflate and begin to function in gas exchange with lung inflation completed
by two weeks of age
 The presence of surfactant, a fatty molecule made by the cuboidal alveolar cells that
lowers the surface tension of the film of water lining each alveolar sac so that the
alveoli do not collapse between each breath is only present in large enough amounts
late in pregnancy – between 28 and 30 weeks
o The respiratory rate is highest in newborns and it continues to drop throughout life until old
age when it can increase again
o The lungs continue to mature throughout childhood and more alveoli are formed until young
adulthood